WASHINGTON — The Trump administration took another swipe at the Affordable Care Act on Tuesday, proposing new rules that would make it much easier for consumers to buy less expensive health insurance policies that do not comply with coverage requirements of the law.

Under current rules, such “short-term, limited-duration insurance” cannot last for more than three months. Under the proposal, the limit would be 364 days.

The move carries out an executive order by President Trump, who said the change would benefit consumers because “short-term, limited-duration insurance is exempt from the onerous and expensive insurance mandates and regulations” in the Affordable Care Act.

Alex M. Azar II, the new secretary of health and human services, said the proposed rules would provide additional options to people who could not afford to pay current insurance premiums.

“Americans need more choices in health insurance so they can find coverage that meets their needs,” Mr. Azar said Tuesday. “The status quo is failing too many Americans who face skyrocketing costs and fewer and fewer choices.”

Short-term policies are intended for people who are between jobs or need temporary coverage for other reasons. They are generally cheaper than insurance that meets the law’s requirements, but they offer significantly less protection to consumers.

Insurers often deny short-term policies to people with pre-existing conditions and can charge higher premiums because of such conditions. Short-term policies do not have to provide the “essential health benefits” that are required by the Affordable Care Act. They may, for example, omit coverage of maternity care, mental health care or addiction treatment. Short-term policies may impose limits on the amount the insurer will pay, but they do not have to cap the patient’s out-of-pocket costs.

In addition, people who buy short-term medical coverage are not eligible for the subsidies provided to low- and middle-income people who buy insurance through the marketplaces set up under the Affordable Care Act.

Democrats denounced the proposal as another attempt by Mr. Trump to sabotage the Affordable Care Act. Republicans called it a potential lifeline for victims of the law who have been priced out of the market.

Senator Ron Wyden of Oregon, the senior Democrat on the Finance Committee, said the Trump administration was promoting “junk insurance.” The House Democratic leader, Representative Nancy Pelosi of California, said people buying these plans could be “one diagnosis away from disaster, discovering they have been paying for coverage that may not cover basic care such as cancer treatment.”

Some health policy experts say that if large numbers of healthy people buy short-term coverage, it could drive up premiums for those who remain in Affordable Care Act plans.

The Trump administration said these concerns were unfounded. But in a preamble to the proposed rules, it said, “Individuals who are likely to purchase short-term, limited-duration insurance are likely to be relatively young or healthy.”

And if those consumers develop chronic conditions, it said, they could “face financial hardship” until they enroll in more comprehensive plans that comply with the 2010 law.

Some insurers worry that consumers will be confused and may see short-term policies as a substitute for major medical coverage.

“Expanded use of short-term policies could further fragment the individual market, which would lead to higher premiums for many consumers, particularly those with pre-existing conditions,” said Kristine Grow, a spokeswoman for America’s Health Insurance Plans, a trade group for insurers.

But Representative Greg Walden, Republican of Oregon and the chairman of the Energy and Commerce Committee, welcomed the announcement as an important step to “expand consumer choice, competition and access to health care.”

Seema Verma, the administrator of the federal Centers for Medicare and Medicaid Services, said that “we don’t think there’s any validity” to concerns that the proposed rules would destabilize insurance markets.

Rather, she said, the short-term policies could appeal to healthy people who are “sitting on the sidelines without coverage.”

The administration estimates that short-term plans may cost about one-third as much as unsubsidized comprehensive coverage.

Under the proposed rules, short-term policies would have to carry a disclosure: “This coverage is not required to comply with federal requirements for health insurance, principally those contained in the Affordable Care Act. Be sure to check your policy carefully to make sure you understand what the policy does and doesn’t cover.”

People will have until April 23 to comment on the proposal. Mr. Azar said he hoped to issue final rules by “mid- to late spring.”

The Trump administration predicted that issuers of short-term insurance would experience “an increase in premium revenues and profits” because they could set prices to reflect the risk that an individual consumer would have high medical costs.

Three cabinet departments — Treasury, labor and health and human services — drafted the proposed rules after Mr. Trump’s order in October, when he said he was saving people from “the nightmare of Obamacare.”

“Today’s action represents an important promise kept by the president,” Mr. Azar said Tuesday.

Senator Ron Johnson, Republican of Wisconsin, said the short-term policies provided a useful alternative to costly major medical coverage that complied with the 2010 health law. Consumers, he said, need “more, not fewer, health insurance options.”

But Mike Kreidler, the state insurance commissioner in Washington State, said short-term medical plans posed risks for consumers.

“If you get sick,” Mr. Kreidler said, “you may not be able to renew your coverage — there are no protections for people with pre-existing conditions. Maternity care and mental health benefits often are excluded.”

Short-term insurance had long been defined, by states and the federal government, as coverage lasting less than 12 months. But the Obama administration changed that in late 2016, after concluding that some insurers were abusing short-term plans and keeping healthier consumers out of the Affordable Care Act marketplaces.

Trump administration officials said that short-term insurance policies had become increasingly important because premiums more than doubled, on average, from 2013 to 2017 for health plans on the federal insurance exchange.

A version of this article appears in print on , on Page A19 of the New York edition with the headline: Trump Moves to Relax Rules on Short-Term Health Insurance. Order Reprints | Today’s Paper | Subscribe